The 1st answer is incorrect. See the specs in the description: Listed to support FIXTURES up to 50 lbs. Listed to support fans up to 35 lbs.
It depends on the fixture you are using, so the maximum wattage would come from those specifications.
The box should have come with (two) #8x1 screws that screw through the two holes in the top wall of the ceiling fan box and into the 2" x 4". in addition, the ceiling fan mounting plate mounts to the bottom of the ceiling fan box using the (two) #8x 3-1/2" screws also included.
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Four inch ceiling boxes, as described, do have two mounting hole locations. One set at 4" centers, and one at 3" centers.